The present invention generally relates to ceramic matrix composite (CMC) materials. More particularly, this invention is directed to methods of forming a CMC article having desired shape, dimensional, and surface characteristics, such as those required to produce a suitable bonding surface.
CMC materials generally comprise a ceramic fiber reinforcement material embedded in a ceramic matrix material. The reinforcement material serves as the load-bearing constituent of the CMC, while the ceramic matrix protects the reinforcement material, maintains the orientation of its fibers, and serves to dissipate loads to the reinforcement material. Of particular interest to high-temperature applications are silicon-based composites, such as silicon carbide (SiC) as the matrix and/or reinforcement material. SiC fibers have been used as a reinforcement material for a variety of ceramic matrix materials, including SiC, TiC, Si3N4, and Al2O3. Continuous fiber reinforced ceramic composite (CFCC) materials are a type of CMC that offers light weight, high strength, and high stiffness for a variety of high temperature load-bearing applications. A CFCC material is generally characterized by continuous fibers (filaments) that may be arranged to form a unidirectional array of fibers, or bundled in tows that are arranged to form a unidirectional array of tows, or bundled in tows that are woven to form a two-dimensional fabric or woven or braided to form a three-dimensional fabric. For three-dimensional fabrics, sets of unidirectional tows may, for example, be interwoven transverse to each other.
Various techniques may be employed in the fabrication of CMC components, including chemical vapor infiltration (CVI) and melt infiltration (MI). Each of these fabrication techniques have been used in combination with tooling or dies to produce a near-net-shape article through processes that include the application of heat and chemical processes at various processing stages. In the fabrication of SiC/Si—SiC (fiber/matrix) CFCC materials disclosed in commonly-assigned U.S. Pat. Nos. 5,015,540, 5,330,854, 5,336,350, 5,628,938, and 6,024,898 and commonly-assigned U.S. Patent Application Publication No. 2004/0067316, continuous SiC-containing fibers or tows are coated to impart certain desired surface properties, such as with an interfacial release agent (e.g., boron nitride or carbon) to allow for limited and controlled slip between adjacent fibers, tows, and the surrounding ceramic matrix. In the case of a two-dimensional fabric, the relatively pliable fabric is cut and shaped within appropriate tooling prior to depositing coatings on the fabric. The tooling is then placed in a CVI reactor, where the desired coatings are deposited to yield a porous fiber preform. To increase the rigidity of the preform, the preform may be further coated with a ceramic material. For example, SiC preforms used in the fabrication of SiC/Si—SiC CMS's may be rigidized by depositing a SiC coating on the tows. The rigidized porous fiber preform is then infiltrated to fill the porosity in the preform, such as slurry casting an aqueous suspension of SiC particles followed by melt infiltration with molten silicon. Melt infiltration of the preform is performed to yield a near net-shape CMC article. During melt infiltration, the molten silicon reacts to form a SiC matrix containing some free silicon.
Within the tooling, the rigidity of the preform can be such that the reinforcement fabric is compressed against the tooling surface, with the result that an imprint pattern of the fabric is often visible on the surface of the final CMC article. Such a defect has been observed with preforms rigidized by CVI-deposited SiC coatings. Depending on the intended application of the article, an imprint pattern may be unacceptable, such as where the article is required to have an optical surface, defined herein as a surface that is sufficiently smooth to be suitable for bonding to, for example, a silicon wafer. Attempts have been made to cover the optical surface of a CMC article marred by an imprint pattern by depositing and then firing a layer of a particulate slurry. However, such attempts often do not produce acceptable results because the resulting coating is very fragile and thus difficult to accurately machine for the purpose of establishing the desired shape and dimensions of the CMC article and its optical surface. To address this problem, others have suggested applying a particulate slurry coating on slurry-cast CMC's to form an extension of the particulate used in the slurry casting process. However, the slurry is too soft to permit machining to high tolerances.
In view of the above, it would be desirable if an improved method were available for producing a CMC article with a surface whose shape, dimensions, and surface finish can be carefully controlled.